1. Technical Field of the Invention
This invention relates to film capacitors; and more particularly to polypropylene film capacitors for use in high temperature applications.
2. Background Art
Polypropylene is an extremely versatile polymer. It is used in both plastics and fiber. It has a melting temperature of 160xc2x0 C., (433.5 K or 320xc2x0 F.). Structurally, it is a vinyl polymer, and is similar to polyethylene, only that on every other carbon atom in the backbone chain has a methyl group attached to it. Polypropylene can be made from the monomer propylene by Ziegler-Natta polymerization and by metallocene catalysis polymerization. 
Capacitors fabricated by prior art methods using known dielectric films composed of polypropylene have maximum operating temperatures approximately between +85xc2x0 and +105xc2x0. Above these temperatures, other types of capacitors must be used, such as those using high temperature polymer films, ceramic, wet and dry tantalum, mica, and aluminum electrolytic capacitors, which are much more costly to produce.
Some examples of patent art providing useful context for understanding the invention are: Ueda""s U.S. Pat. No. 6,094,337, Polypropylene Film and Capacitor Made by Using it as a Dielectric; Tamic""s U.S. Pat. No. 6,127,042, Capacitor having a Polypropylene Dielectric, and a Metallized Film for making such a Capacitor, and Sato""s U.S. Pat. No. 4,185,148, Process for Producing the Polypropylene Film for Electrical Appliances. 
As Ueda states, xe2x80x9c. . . the polypropylene film wound with an electrode is generally annealed at a certain temperature, and thereby moderately thermally shrunk for tightening the winding to maintain its shape and to squeeze out the air between film layers, and in this case if the heat shrinkage is too large, the device may be deformed, lowering the capacity of the capacitor, or it may be destroyed. If the heat shrinkage is too small, the tightening of the winding may be insufficient, or the dielectric loss may increase to destroy the device when used at high temperature for a long time.xe2x80x9d Thus, if the temperature is too high or the capacitor is exposed to the high temperature for too long, the polypropylene anneals to such an extent that the capacitor is compromised. Ueda discloses the use of a biaxially oriented film with improved dielectric properties, but nowhere questions or challenges conventional operating temperature limits of 105 degrees Centigrade.
Various commercial sources of electronics components offer polypropylene film capacitors, all listing an upper temperature limit of 105 degrees Centigrade. Higher temperature capacitors such as ceramic and tantalum being significantly more expensive, clearly there is a need for a process that would extend the useful operating temperature range of the relatively inexpensive polypropylene film type capacitors while maintaining their physical and electrical properties.
It is an object of the present invention to provide metallized polypropylene film capacitors that are capable of a useful operating life at a temperature greater than 105xc2x0 C., and even greater than 125xc2x0 C.
It is a further object to provide a method for production of polypropylene film capacitors with higher than present operating temperature limits, using conventional materials and equipment. It is yet another object of the invention to provide such capacitors while maintaining high stability, high insulation resistance, and low losses. It is a yet further object of the invention to provide such a capacitor having low dielectric absorption and excellent AC performance.
It is another object of the invention to provide a monolithic polypropylene film capacitor of higher than previous operating temperatures by the process comprising the steps of: vapor depositing metal electrodes on polypropylene dielectric film; winding the film; applying solderable or weldable spray material to the end of each of the electrodes using metallizing equipment, thereby forming a capacitor; and then slowing heating the capacitor in an evacuated oven to a peak temperature of about 10 to 20 degrees higher than the intended operating temperature, the peak temperature being between about approximately 115xc2x0 C. (388.15 K) and an upper limit of approximately 200xc2x0 C. (473.15 K) and maintaining the peak temperature for a period of time.
It is still another object of the invention to provide such a monolithic polymer capacitor wherein said polypropylene dielectric film is commercially available. It is yet another object of the invention to provide such a monolithic polymer capacitor wherein said capacitor has a capacitance of between 0.001 and 800 microfarads (xcexcF).
It is a still further object of the invention to provide such a monolithic polymer capacitor wherein said capacitor is heated at an average rate over the curing process of not more than about approximately 5xc2x0 C./hour, up to a peak temperature of not more than 200xc2x0 C., and preferably not more than about 180xc2x0 C. It is a yet further object of the invention to provide such a monolithic polymer capacitor wherein the final peak temperature curing time is not less than about approximately 1 hour. It is still another even further object of the invention to provide such a monolithic polymer capacitor wherein the capacitor has a maximum operating temperature limit greater than about approximately 105xc2x0 C. (378.15 K), and preferably at least about 125xc2x0 C., which is at least about 10 degrees lower than the peak curing temperature, and preferably about 20 degrees or more lower.
It is another further object of the invention to provide such a monolithic polymer capacitor wherein the step of winding further comprises using a standard capacitor winding machine. It is still another further object of the invention to provide such a monolithic polymer capacitor wherein said capacitor winding machine has continuous tension control.
Other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description, wherein we have shown and described only a preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by us on carrying out our invention.